Friday 16 September 2005 19.14 EDT
First published on Friday 16 September 2005 19.14 EDT

She May Not Leaveby Fay Weldon288pp, Fourth Estate, £15.99

Fay Weldon's 25th novel fictionalises a phenomenon of contemporary family life mainly confined to headlines - the presence within the bosom of the family of an ambiguous live-in employee, the nanny or au pair. Sleeping with the husband (or ex-husband), charged with abusing the child, revealing a fake identity, spying on her employers and selling their family secrets to the tabloids or turning them into comic bestsellers such as The Nanny Diaries, these young women have earned a place among the dramatis personae of serious contemporary fiction as well.

Unlike the elderly nannies of Victorian legend, the modern nanny is young and beautiful; and her sexual attraction makes her a rival. Yet her childcare skills often surpass those of the biological mother, and, as the French used to say about marriage, the burden of modern parenting is so heavy that it takes three to carry it. Thus the au pair is essential to the smooth functioning of the contemporary family of working parents and contented children; as Weldon suggests in the title of the book, she may not leave.

The novel begins with Martyn and Hattie, unmarried but committed working parents, employing a Polish au pair, Agnieszka, to care for their baby, Kitty. Although they had planned to share the parenting, Martyn finds he cannot get away from his work at a liberal political magazine called Devolution, while Hattie yearns to return to her job as a literary agent. Both strive for what some call political correctness but Weldon calls the moral high ground - support for immigrants and asylum seekers, distaste for competition, belief in religious secularism and gender equality. But, as Hattie discovers, "morality is a question of what one can afford". All their values are under pressure, and they have to compromise, or admit their own hypocrisy, at home and at work. When Agnieszka arrives, she is like a "fairy godmother", bringing order out of chaos, peace out of strife, but alas, also voluptuousness. She is Catholic, has studied child development, loves kittens, and takes classes in belly-dancing. And of course, she is not what she seems to be.

Weldon adds to this satirical mix a complex narrative structure in which the story is told by Hattie's 72-year-old grandmother Frances Watt, who also tells the stories of her novelist sister Serena (a Weldon-like figure) and her daughter Lallie. This device allows Weldon to comment on the different attitudes of child-rearing and working mothers over generations, and to let her astonishing imagination create yet another array of couples, careers, houses, love affairs and children, all punctuated by her aphoristic wit: "guilt is to motherhood as grapes are to wine".

Frances warns the reader about Hattie's unmarried status, and the addition to her household of a new woman from the new Europe: "She will be more desperate for survival; the cultures where men looked after women are vanishing fast." We are offered cautionary tales from previous generations. Still, when Agnieszka makes her move, her tactics are shocking, and the last third of the novel offers some swift, sharp turns. Weldon saves until the very end a moral twist, a discovery in key with her other works, but one that still surprises. This prolific and very successful writer still has many tricks up her designer sleeve.